change from mass to molar units

Dear all, I have fitted a simple two-compartment model with linear elimination to some compound data (ADVAN4 TRANS4). Everything worked well. Because I want to extend this model to account for metabolite data as well, I now wanted to change all data to molar units, i.e. I changed the AMT and DV column in the same way. To my understanding, if I change both these columns, the obtained parameters should end up being the same. However, when I use mg and µg/mL the model runs just fine, whereas when I use µmol and nmol/L the parameter estimates change completely and always run into some boundary. Can anyone shed some light on why this happens? Is there any way to get around this? Could I keep the mass units and do some changes in the control stream when incorporating the metabolite data? Any help would be highly appreciated. Best regards Nele _________________________ Nele Plock Bayer Schering Pharma AG GPD/Pharmacokinetics Metabolism & Bioanalysis D- 13342 Berlin Phone : +49-30-468 15146 Fax: +49-30-468 95146 [EMAIL PROTECTED]

Nele, Two suggestions: 1. You have mass units of mg and mg/L but molar units of nmol/1000 and nmol/L. So the ratio of AMT to DV is different by a factor of 1000 2. If you have an additive error model then the scale of the residual error model parameter will need to change. Best wishes, Nick [EMAIL PROTECTED] wrote: > > Dear all, > > I have fitted a simple two-compartment model with linear elimination to > some compound data (ADVAN4 TRANS4). Everything worked well. Because I want > to extend this model to account for metabolite data as well, I now wanted > to change all data to molar units, i.e. I changed the AMT and DV column in > the same way. To my understanding, if I change both these columns, the > obtained parameters should end up being the same. However, when I use mg > and µg/mL the model runs just fine, whereas when I use µmol and nmol/L the > parameter estimates change completely and always run into some boundary. > Can anyone shed some light on why this happens? Is there any way to get > around this? Could I keep the mass units and do some changes in the > control stream when incorporating the metabolite data? Any help would be > highly appreciated. > > Best regards > Nele > > _________________________ > Nele Plock > Bayer Schering Pharma AG > GPD/Pharmacokinetics > Metabolism & Bioanalysis > D- 13342 Berlin > > Phone : +49-30-468 15146 > Fax: +49-30-468 95146 > [EMAIL PROTECTED] -- Nick Holford, Dept Pharmacology & Clinical Pharmacology University of Auckland, 85 Park Rd, Private Bag 92019, Auckland, New Zealand email:[EMAIL PROTECTED] tel:+64(9)373-7599x86730 fax:373-7556 http://www.health.auckland.ac.nz/pharmacology/staff/nholford/